The present invention is directed to a novel process for preparing 6-substituted-2-vinylnaphthalene. In particular, the invention is directed to preparing 6-substituted-2-vinylnaphthalene from 2-substituted naphthalene and 6-substituted-2-acylnaphthalene.
6-substituted-2-acylnaphthalenes have use as precursors for aromatic monomers used in the formation of polymers, pharmaceuticals, and other value-added chemicals. There is much discussion in both the patent and the technical literature relative to producing the 6-substituted-2-acylnaphthalenes. Typically, the 6-substituted naphthalenes are acylated in the presence of a Friedel-Crafts catalyst. Early improvements in this field were based on attempts to find useful solvents for the Friedel-Crafts reaction. An example is U.S. Pat. No. 3,234,286 which describes the replacement of the very toxic nitrobenzene solvent with 2-nitro propane. Typically, the literature contained general comments that the Friedel-Crafts type acylation of 2-substituted naphthalenes could be carried out in the presence of any Friedel-Crafts catalyst. Usually the general statement was substantiated by particular reference to experiments using, e.g., aluminum chloride or zinc chloride catalyst in nitrobenzene or chlorinated hydrocarbons, or boron trifluoride optimally in the presence of hydrogen fluoride. Most often, the literature did not identify the position of the acyl substitution on the 2-substituted naphthalene ring or did not provide such substitution in the 6-position with sufficient selectivity that a large scale manufacturing program could be developed using the disclosed technology.
Using the teachings of the prior art, commercialization of the process has been difficult. Aluminum chloride, zinc chloride, and similar salts used as Lewis acid catalysts in their anhydrous state absorb water rapidly, hydrolyzing, and thus becoming useless in the reaction. Nitrobenzene and the polyhalogenated hydrocarbons utilized as solvents are high boiling, toxic materials, and as such are undesirable solvents.
Nitrohydrocarbons, in the presence of the water invariably required during the usual work-up, give rise to inseparable emulsions. Also, the usual Friedel-Crafts catalysts, when admixed with nitrohydrocarbons, generate a substantial exothermic reaction which can be difficult to control. Conversions are low. When using metal halides in nitrohydrocarbons, isomeric mixtures are common, causing additional separation steps to be necessary.
U.S. Pat. No. 4,593,125 relates to an improved process for the acylation of naphthalene substituted in the 2-position by an electron-donating substituent. The inventors' purpose was to develop precursors for aromatic monomers for polyesters, pharmaceuticals, and other value-added chemicals by obtaining nearly exclusively 6-substituted-2-acylnaphthalenes in high conversions with high regioselectivity. In the process disclosed in this mentioned patent, the naphthalene compound to be acylated is brought in contact with substantially anhydrous hydrogen fluoride and the acylating agent for a time sufficient to cause substantially complete conversion of the naphthalene compound to a 6-substituted-2-acylnaphthalene.
6-substituted-2-vinylnaphthalene also has prospective use as a monomeric precursor in the formation of aromatic polymers, as well as having pharmaceutical applications and applications as a precursor for various chemicals. A proposed route to forming vinylnaphthalene from ethylnaphthalene is disclosed in U.S. Pat. No. 2,468,759. As disclosed therein, ethylnaphthalene is oxidized to a mixture of methyl naphthyl carbinol and methyl naphthyl ketone. The methyl naphthyl ketone is hydrogenated to methyl naphthyl carbinol and the methyl naphthyl carbinol dehydrated to vinylnaphthalene. There is no disclosure of forming a 6-substituted-2-vinylnaphthalene in this patent. Moreover, it is not merely a simple procedure to hydrogenate the naphthone to the corresponding carbinol over broad conditions and any hydrogenation catalyst as over-hydrogenation to the ethylnaphthalene readily results. In addition, the dehydration reaction must be controlled by use of process conditions and type of catalyst to prevent or to minimize the polymerization of the formed vinylnaphthalene.
Accordingly, it is an object of the present invention to provide an improved process for the production of 6-substituted-2-vinylnaphthalene.